Ethylene polymers have heretofore been widely used as materials of hollow molded articles, extrusion molded articles, films and sheets. The properties required for such ethylene polymers vary depending on the use of the polymers and the molding method thereof. For example, in order to prevent vibration or the bursting of bubbles at the time of molding inflation films at high speed, or to prevent drawdown at the time of hollow molding or sheet forming, an ethylene polymer having a high melt tension must be used. Further, in order to produce bottles having a good shape of pinch-off scar or hollow molded articles having a narrow thickness distribution by hollow molding, an ethylene polymer having a high swell ratio must be used.
Ethylene polymers prepared by the use of Ziegler-Natta catalysts, such as MgCl.sub.2 -carrier support type Ti catalysts, are excellent in rigidity and impact resistance because almost no long-chain branch is present therein, but they are inferior in moldability to ethylene polymers prepared by the use of Cr type Phillips catalyst. On the other hand, the ethylene polymers prepared by a high-pressure method or prepared by the use of the Cr type Phillips catalysts have higher melt tension, higher swell ratio and more excellent moldability than the ethylene polymers prepared by the use of the Ziegler-Natta catalysts, but they deteriorate in rigidity and impact resistance due to the presence of the long-chain branches.
Under these circumstances, there have been made various studies on the ethylene polymers which are prepared by the use of Ziegler-Natta catalysts and are excellent in moldability and the like.
For example, Japanese Patent Laid-Open Publication No. 12735/1980 describes an ethylene polymer which is obtained by blending an ethylene polymer prepared by the use of Ziegler-Natta catalyst with an ethylene polymer prepared by means of a high-pressure method. Further, Japanese Patent Laid-Open Publication No. 36546/1985 describes an ethylene polymer which is obtained by blending an ethylene polymer prepared by the use of Ziegler-Natta catalyst with an ethylene polymer prepared by the use of Cr type Phillips catalyst. These ethylene polymers are improved in moldability, and however, the rigidity and impact strength inherent in the ethylene polymer prepared by the use of Ziegler-Natta catalyst are deteriorated because the proportion of the long-chain branches is increased.
Further, Japanese Patent Laid-Open Publication No. 89341/1984 describes an ethylene polymer which is obtained by modifying an ethylene polymer prepared by the use of Ziegler-Natta catalyst, in the presence a radical initiator; and Japanese Patent Laid-Open Publication No. 164347/1984 describes an ethylene polymer which is obtained by modifying an ethylene polymer prepared by the use of Ziegler-Natta catalyst, in the presence of maleic acid. Also in these cases, these ethylene polymers are improved in moldability, and however, the rigidity and impact strength thereof are deteriorated because the proportion of the long-chain branches is increased.
Furthermore, Japanese Patent Laid-Open Publications No. 158204/1982 and No. 106806/1985 describe ethylene polymers which are obtained by the use of a specific catalyst, have a small proportion of the long-chain branches and are excellent in the moldability. However, these ethylene polymers have large values of Mw/Mn and Mz/Mw and include a polymer having a molecular weight extremely larger than the average molecular weight. Such a polymer having a molecular weight extremely larger than the average molecular weight causes poor appearance of its molded article such as fish eye in many cases.
Still further, Japanese Patent Laid-Open Publication No. 130314/1986 describes an ethylene polymer excellent in moldability which is prepared by a multi-stage polymerization process. This ethylene polymer has a broad molecular weight distribution thereby to include an ethylene polymer having a molecular weight extremely larger than the average molecular weight, resulting in poor appearance of the molded article such as fish eye.
As described above, there has been earnest research on the ethylene polymers which are prepared by the use of Ziegler-Natta catalyst, are excellent in moldability and can be molded into articles excellent in mechanical strength and almost free from poor appearance, but any ethylene polymer having such properties was not found.